The nonlinear noise of a fibre channel mainly comprises the self-phase modulation (SPM) noise caused by the fluctuations of the signal power of its own channel, and cross-phase modulation (XPM) noise caused by the fluctuations of the signals power of adjacent channels. For the self-phase modulation noise generated by a signal after being transmitted in a span of fiber, its numerical value may be calculated based on the power of the signal transmitted in this span of fiber. And for the self-phase modulation noise of the whole transmission link, due to the fiber dispersion in the transmission link, the power of the signal being transmitted changes along with the transmission distance, so it is necessary to divide the whole transmission link into multiple spans and seriatim calculate the corresponding span, with the calculation precision being dependent on the granularity of the spans. The finer the granularity is, the higher the precision of the calculation, but the higher the complexity of the calculation.